The present invention relates to tubing circuit systems for delivering humidified respiratory gas to a human or an animal. More particularly, the invention relates to such tubing circuit systems which are straight forward in design and construction, and very effectively and reliably monitor and/or control the temperature of the humidified respiratory gas delivered to the human or animal, even under adverse or abnormal conditions.
Ventilators, with a humidifier typically at the outlet of the ventilator, supplying gas mixtures of air and one or more gaseous components, for example, oxygen, anesthesia and the like, to patients, usually in hospitals, do so through what is known as a "patient circuit". Such patient circuits conduct the gases out to a patient manifold and back to the ventilator's exhalation valve which closes and opens, raising and lowering the pressure to the patient's lungs in the typical ventilator arrangement.
Additionally, the patient circuits often have either water traps to collect condensate from the humidifiers in the inspiratory and expiratory flow tubes or they have heating wires in or around the tubes to minimize the condensation. When the circuit is heated, temperature sensors are included and are coupled to controllers to control the amount of heat provided to the patient circuit, and the humidifier's patient temperature probe, which is normally at the patient manifold, is moved back to the outlet of the humidifier. Most circuits also include a patient pressure sensing line connected to a port on or near the patient manifold. Sometimes other gas sampling and atomized drug treatment and suctioning ports exist for specialized occasional measurements or treatments.
Although heated patient circuits eliminate the need for water traps in the tubing legs, the prior art heated circuits do have certain problems and disadvantages. For example, because of the structure and/or location or placement of the temperature sensors in the prior art circuits, there is a relatively high propensity for such circuits to deliver respiratory gas which is either warmer (over temperature) or cooler (under temperature) than desired. Such warmer or cooler respiratory gas can result in patient discomfort and irritation, and even in severe respiratory system damage, particularly when the patient is a human infant or child, for example, under one year of age. In addition, these prior art heated patient circuits are relatively "unforgiving" of user or operator errors. For example, serious over temperatures or under temperatures are quite likely to occur if such prior art circuits are accidentally used backward, that is with the inspiratory line used as the expiratory line, and vice versa. Of course, it would be advantageous to provide a circuit system which reduces the severity, or even overcomes, one or more of these problems or concerns.
Elsworth et al U.S. Pat. No. 4,708,831 discloses a humidifier and circuit for delivery of humidified gases to a patient. Thermistors are provided in the inspiratory line and provide signals externally of the circuit to a control system. Also, two separate heating elements are provided, one in the inspiratory line and another in the expiratory line. Only the heating element in the inspiratory line is controlled by the control system. The systems disclosed in this patent are subject to temperature sensing errors, for example, because of heat losses due to the temperature signals being transported outside of the circuit. Also, slight reverse gas flows and/or convection currents can result in gas at a higher than desired, or even uncontrolled, temperature, (for example, from the expiratory line) being passed to the patient.
Systems which employ conventional external temperature probes are also susceptible to temperature sensing errors. For example, such probes can be sensitive to the temperature of the environment external to the circuit system, such as in an incubator (or at an incubator wall) and/or under a warmer, rather than accurately sensing the temperature of the gas in the circuit system.
French Patent No. 2,250,042 discloses a temperature detector at the end of a tubular humidifier heater. However, since the detector is located at one end of the heater, zero gas flow and convection currents can affect the reading of the detector, and small negative or reverse flows can inactivate the detector. Also, this French Patent concerns a humidifier, and not a heated patient circuit.
It would be advantageous to provide tubing circuit systems for delivery of humidified respiratory gas in which gas heating can be conveniently and reliably maintained, even under adverse or abnormal conditions, and/or in which temperatures can be accurately and reliably sensed and controlled.